1. Field of the Invention
The present invention generally relates to a liquid crystal display device and, more particularly, to a thin film transistor liquid crystal display device with capacitors having high capacitance.
2. Description of the Related Art
Liquid crystal display devices are generally classified into two categories: i.e. the passive matrix liquid crystal display and the active matrix liquid crystal display. For most of the active matrix liquid crystal display devices, the brightness of these devices is controlled by the switches (i.e. the thin film transistors) and the auxiliary storing capacitors of pixels. However, the size of the area of the switches and the auxiliary storing capacitors in the pixels will affect the size of the aperture ratio of each pixel. The size of the aperture ratio is an important factor for the brightness of the pixel or even the display device. Hence, the smaller the size of the total area of the switch and the auxiliary storing capacitor in each pixel is, the higher the aperture ratio of the display device will be.
When the size of the auxiliary storing capacitor is reduced, it is difficult to maintain enough capacitance on the tiny-sized capacitors to control the elements of a pixel. Moreover, the thickness of the capacitors, the properties of the capacitors, and even the size of the capacitors can also affect the storing of the charges and the voltage applied on the pixel electrodes. Hence, any solution that can increase the aperture ratio without reducing the storing charge of the storing capacitors is greatly needed. So far, the suggestion for increasing the stored charge of the capacitor per unit area directly is the most economic way to achieve the goal illustrated above. Moreover, increasing the stored charge of the capacitors can reduce the area of the storing capacitors in the pixels, increase the aperture ratio, and improve the display quality.
So far, most researchers achieve the increase of the stored charge of the capacitors through increasing the surface area of the capacitor. Many researchers suggest transferring the plane surface of the original capacitors into a rugged surface for increase the surface area in a limited space. For example, a method for forming an organic layer with rugged surface on the substrate is disclosed in U.S. Pat. No. 4,106,859. The method disclosed in U.S. Pat. No. 4,106,859 is achieved by impressing, just as shown in FIG. 1A. The original organic layer 120 with plane surface is formed on a substrate first. The organic layer 120 is then impressed by a head 110 with a rugged or wave-like surface and a pressure P to form a rugged surface on the surface of the organic layer. Alternatively as shown in FIG. 1B, an organic layer 120 with a rugged surface forms after the original organic layer 120 is impressed by rolling a cylinder with a rugged or wave-like surface. In addition, another method for manufacturing rugged surface on a substrate without impressing is disclosed by Yoshiaki et al. in U.S. Pat. No. 4,519,678. As shown in FIG. 2, Yoshiaki et al. forms several patterned bumps 510 on a substrate 500 directly first. Then a resin layer 520 and a metal layer 530 is coated over the patterned bumps 510 in sequence to form a rugged surface over or on the substrate. However, since several steps for forming layers are needed, the method disclosed by Yoshiaki et al. is too complicate to mass-produce the substrate for display devices.
There is thus a general need for a method for manufacturing a rugged lower plate of a liquid crystal display device.